Solid electrolyte material and method for producing the same

ABSTRACT

In order to improve the stability of an electrolyte, an object of the present disclosure is to develop, among the sulfide solid electrolytes of Li—P—S—O based containing no metal element other than lithium, a new solid electrolyte having a possibility to have high ion conductivity and a method for producing for obtaining the same easily. The present disclosure achieves the object by providing a solid electrolyte material including a sulfide composition represented by a composition formula Li4-4y-xP4+1+y-xP5+xS4-zOz (Li4-4y-xP1+yS4-zOz), wherein 0.6≤x&lt;1, 0≤z≤0.2, and −0.025≤y≤0.1, and a method for producing the same.

TECHNICAL FIELD

The present disclosure relates to a solid electrolyte material and amethod for producing the same, particularly, to a solid electrolytematerial including a Li element, a P (IV) element, a P (V) element, a Selement, and an O element.

BACKGROUND ART

In accordance with higher performance of electronic informationapparatuses such as a portable telephone, a notebook personal computer,and a tablet personal computer in recent years, a high performancebattery for operating these electronic information apparatus for a longtime by a single battery charge is desired. Also, for reducing thegreenhouse gases and due to rising gasoline prices, hybrid vehicles andelectric vehicles became rapidly widely used so that high power and highcapacity batteries for operating the motors loaded on these vehicles aredesired. As batteries fulfilling such demands, lithium batteries aremainly used currently.

As electrolytes for lithium batteries, flammable organic solvents arecurrently used for the reasons such as high ion conductivity, a widepotential window, and low cost. However, since the energy density of thelithium batteries are extremely high, the flammable organic solvents arenot preferable in light of safety. In order to further improve thesafety of the lithium batteries, flame-resistant materials are desirablyused for the electrolytes of the lithium batteries. As suchflame-resistant materials, inorganic solid electrolytes attractattention.

As for the inorganic solid electrolyte, there are inorganic electrolytesof amorphia such as nitride, oxide, and sulfide; and of crystalline. Thefollowings are known as the sulfide glass solid electrolyte: athree-component glassy solid electrolyte of lithium sulfide, germaniumdisulfide, and lithium iodide (Patent Literature 1), and a solidelectrolyte wherein lithium phosphate exists in lithium ion conductivesulfide glass represented by a general formula Li₂—X (Patent Literature2). The ion conductivities of these are at a level of 10⁻⁴ S/cm.Further, instead of the amorphia, as the crystalline material, acrystalline material having a tetrahedron basic structure of SiS₄, PO₄,PS₄ or PN₄ is being researched in hopes of high ion conductivity, andthe ion conductivity in a range of 10⁻⁵ S/cm to 10⁻⁴ S/cm is reportedfor a Li₂S—GeS₂—Ga₂S₃ based solid electrolyte (Patent Literature 3).

Among the solid electrolytes, as a solid electrolyte with extremely highlithium ion conductivity, a sulfide solid electrolyte called sulfidethio-LISICON (thio-LISICON: LIthium SuperIonic CONductor) is known.Among them, the ion conductivity of Li_(3.25)Ge_(0.25)P_(0.75)S₄ is2.2×10⁻³S/cm, and is the highest among the sulfide thio-LISICON (forexample, refer to Non-Patent Literature 1). Further, in order to improvethe stability of electrolytes, Li—P—S based and Li—P—S—O based sulfidesolid electrolytes are reported as the sulfide thio-LISICON notincluding a metal element other than lithium (for example, refer toNon-Patent Literatures 2 and 3).

As a solid electrolyte with high conductivity including a Li—P—S—O basedsulfide solid electrolyte, Patent Literature 4 proposes a sulfide solidelectrolyte represented by a composition formulaLi_(3+5x)P_(1-x)PS_(4-z)O_(z), wherein 0.01≤x≤0.105 and 0.01≤z≤1.55.Also, Patent Literature 5 proposes a sulfide solid electrolyte materialincluding a composition of Li_(5x+2y+3)P^((III)) _(y)P^((V)) _(1-x-y)S₄,wherein 0≤x≤0.2 and 0<y≤0.3.

CITATION LIST Patent Literatures

Patent Literature 1: Publication of Examined Japanese Patent ApplicationNo. H06-70906

Patent Literature 2: Japanese Patent No. 3184517

Patent Literature 3: Japanese Patent No. 3744665

Patent Literature 4: Japanese Patent No. 5787291

Patent Literature 5: WO 2014/196442

Non-Patent Literatures

Non-Patent Literature 1: R. Kanno and M. Murayama, “Lithium IonicConductor Thio-LISICON The Li₂S—GeS₂—P₂S₅ System”, Journal of TheElectrochemical Society, 148 (7), A742-A746 (2001)

Non-Patent Literature 2: M. Murayama, N. Sonoyama, A. Yamada and R.Kanno, “Material design of new lithium ionic conductor, Thio-LISICON, inthe Li₂S—P₂S₅ System”, Solid State Ionics, 170, 173-180 (2004)

Non-Patent Literature 3: K. Takeda, M Osada, N. Ohta, T. Inada, A.Kajiyama, H. Sasaki, S. Kondo, M. Watanabe and T Sasaki, “Lithium ionconductive oxysulfide, Li₃PO₄—Li₃PS₄”, Solid State Ionics, 176,2355-2359 (2005)

SUMMARY OF DISCLOSURE Technical Problem

However, in light of improving power of the batteries, solidelectrolytes having higher ion conductivity and are more stable inbattery chemistry are demanded. As mentioned above, for example, a LGPStype sulfide solid electrolyte including Ge is reported to exhibit highion conductivity; however, higher ion conductivity is demanded, andalso, high cost of Ge and low chemical stability such as areduction-resistance are pointed out. Accordingly, in order to improvethe stability of the electrolytes, an object of the present disclosureis to develop, among the Li—P—S—O based sulfide solid electrolytescontaining no metal element other than lithium, a new solid electrolytehaving a possibility to have high ion conductivity, and a method forproducing for obtaining the same easily.

Solution to Problem

In order to achieve the object, the present disclosure employs thefollowing constitutions.

(1) A solid electrolyte material according to the disclosure of claim 1comprising a sulfide composition represented by a composition formulaLi_(4-4y-x)P⁴⁺ _(1+y-x)P⁵⁺ _(x)S_(4-z)O_(z)(Li_(4-4y-x)P_(1+y)S_(4-z)O_(z)), wherein 0.6≤x<1, 0≤z≤0.2, and−0.025≤y≤0.1.(2) The disclosure of claim 2 is the solid electrolyte materialaccording to claim 1, wherein the solid electrolyte material has a peakat a position of 2θ=29.58°±0.50° in X-ray diffraction measurement usinga CuKα ray, and the solid electrolyte material does not have a peak at aposition of 2θ=27.33°±0.50° in X-ray diffraction measurement using aCuKα ray, or when the solid electrolyte material has a peak at theposition of 2θ=27.33°±0.50°, a diffraction intensity of the peak at2θ=29.58°±0.50° is regarded as I_(A), and a diffraction intensity of thepeak at 2θ=27.33°±0.50° is regarded as I_(B), a value of I_(B)/I_(A) isless than 0.50.(3) The disclosure of claim 3 is the solid electrolyte materialaccording to claim 1, wherein the solid electrolyte material has a peakat a position of 2θ=17.90°±0.20°, 29.0°±0.50°, and 29.75°±0.25° in X-raydiffraction measurement using a CuKα ray, and the solid electrolytematerial does not have a peak at a position of 2θ=18.50°±0.20° in X-raydiffraction measurement using a CuKα ray, or when the solid electrolytematerial has a peak at the position of 2θ=18.50°±0.20°, a diffractionintensity of the peak at 2θ=17.90°±0.20° is regarded as I_(C), and adiffraction intensity of the peak at 2θ=18.50°±0.20° is regarded asI_(D), a value of I_(D)/I_(C) is less than 0.50.(4) The disclosure of claim 4 is the solid electrolyte materialaccording to claim 1, wherein the solid electrolyte material has a peakat a position of 2θ=18.00±0.1°, 19.4°±0.1°, 21.9°±0.1°, 24.0°±0.1° and31.3°±0.1° in X-ray diffraction measurement using a CuKα ray.(5) The disclosure of claim 5 is the solid electrolyte materialaccording to claim 1, wherein the solid electrolyte material has a peakat a position of 2θ=17.8°±0.1°, 19.1°±0.1°, 21.7°±0.1°, 23.8°±0.1° and30.85°±0.1° in X-ray diffraction measurement using a CuKα ray.(6) The disclosure of claim 6 is the solid electrolyte materialaccording to claim 1, wherein ion conductivity is 0.4 mS/cm or more.(7) The disclosure of claim 7 is a method for producing the solidelectrolyte material according to claim 1, the method including: an ionconductive material synthesizing step of synthesizing an ion conductivematerial using a simple substance of P, a P compound, a S compound, a Licompound, and an O compound as a raw material including a constituent ofthe sulfide composition; and a heating step of obtaining the sulfidecomposition by heating the ion conductive material.(8) The disclosure of claim 8 is the method for producing the solidelectrolyte material according to claim 7, wherein a heating temperaturein the heating step is in a range of 230° C. to 300° C.

Advantageous Effects of Disclosure

According to the present disclosure, a solid electrolyte materialincluding a Li—P—S—O based sulfide solid electrolyte, and having highion conductivity and high chemical stability may be obtained easily.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a composition diagram of a triangular prism shape ofLi₂S—PS₂—P₂S₅—Li₂O—PO₂—P₂O₅ based showing a composition range of thesulfide solid electrolyte in the present disclosure.

FIG. 2A is a diagram showing the X-ray diffraction peaks found in a newphase A and various crystal structures.

FIG. 2B is a diagram showing the X-ray diffraction peaks found in a newphase B and various crystal structures.

FIG. 3 is a ternary composition diagram of Li_(4-4y-x)P⁴⁺ _(1y-x)P⁵⁺_(x)S_(4-z)O_(z) (Z=0).

FIG. 4 is a ternary composition diagram of Li_(4-4y-x)P⁴⁺ _(1+y-x)P⁵⁺_(x)S_(4-z)O_(z)(z=0.1).

FIG. 5 is a ternary composition diagram of Li_(4-4y-x)P⁴⁺ _(1+y-x)P⁵⁺_(x)S_(4-z)O_(z) (z=0.2).

FIG. 6 is an X-ray diffraction diagram of the sulfide solid electrolytehaving a composition of Li_(4-4y-x)P⁴⁺ _(1y-x)P⁵⁺ _(x)S_(4-z)O_(z)(z=0).

FIG. 7 is an X-ray diffraction diagram of the sulfide solid electrolytehaving a composition of Li_(4-4y-x)P⁴⁺ _(1+y-x)P⁵⁺ _(x)S_(4-z)O_(z)(z=0.1).

FIG. 8 is an X-ray diffraction diagram of the sulfide solid electrolytehaving a composition of Li_(4-4y-x)P⁴⁺ _(1+y-x)P⁵⁺ _(x)S_(4-z)O_(z)(Z=0.2).

FIG. 9 is a diagram showing the X-ray diffraction peaks of eachcomposition, when z is varied from 0 to 0.5 in the composition ofLi₃PS_(4-z)O_(z), next to each other.

FIG. 10 is a diagram showing the X-ray diffraction peaks of eachcomposition, when z is varied from 0 to 0.2 in the composition ofLi_(3.2)P_(0.96)S_(4-z)O_(z), next to each other.

FIG. 11 is a diagram showing the X-ray diffraction peak results of a newphase A and the peaks of the known crystal structures, next to eachother.

FIG. 12A is a diagram showing the X-ray diffraction measurement resultsin replicating tests to confirm the new phase A.

FIG. 12B is a diagram showing X-ray diffraction measurement results inwhich the new phase B was confirmed.

FIG. 13 is a diagram showing the ion conductivity of the solidelectrolyte materials according to the present disclosure together withX-ray diffraction charts thereof.

DESCRIPTION OF EMBODIMENTS

As the result of dedicated researches, the present inventors have foundout that a solid electrolyte material including a sulfide compositionrepresented by a composition formula Li_(4-4y-x)P⁴⁺ _(1+y-x)P⁵⁺_(x)S_(4-z)O_(z), (Li_(4-4y-x)P_(1+y)S_(4-z)O_(z)), wherein 0.6≤x<1,0≤z≤0.2, and −0.025≤y≤0.1 has high ion conductivity and high chemicalstability, and may be easily produced. Thereby, the present disclosurehas been achieved. Although the present disclosure will be hereinafterdescribed in detail, the present disclosure is not limited to thefollowing embodiments.

<Sulfide Solid Electrolyte>

The sulfide solid electrolyte (sulfide composition) in the presentdisclosure will be explained referring to a composition diagram of atriangular prism shape of Li₂S—PS₂—P₂S₅—Li₂O—PO₂—P₂O₅ based shown inFIG. 1. The bottom surface of this triangular prism shaped compositiondiagram is a ternary diagram of a sulfide, and the top surface thereofis a ternary diagram of an oxide; the sulfide component increases towardthe lower side of the triangular prism, and the oxide componentincreases toward the upper side of the triangular prism. In more detail,the sulfide ternary diagram at the bottom surface is a ternary diagramof Li₂S-P⁽⁴⁺⁾S₂-P⁽⁵⁺⁾ ₂S₅(Li—P—S system of lower left of FIG. 1), andthe oxide ternary diagram at the top surface is a ternary diagram ofLi₂O-P⁽⁴⁺⁾O₂-P⁽⁵⁺⁾ ₂O₅ The sulfide solid electrolyte in the presentdisclosure has a composition that is plotted inside (excluding sides)the triangular prism, and the composition formula may be represented byLi_(4-4y-x)P⁴⁺ _(1+y-x)P⁵⁺ _(x)S_(4-z)O_(z)(═Li_(4-4y-x)P_(1+y)S_(4-z)O_(z)). Here, z is a factor relating theratio of S and O. The case where z=0 corresponds to the bottom surface(a sulfide ternary diagram), and the case where z=1 corresponds to thetop surface (an oxide ternary diagram); as z increases from 0, thecomposition includes more oxygen and the plot in the triangular prismmoves toward the top surface. The factors x and y will be explainedreferring to the ternary diagram of Li—P—S—O system shown in the upperleft of FIG. 1. This ternary diagram is a face cut out from thetriangular prism at z selected in the range of 0<z<1, and the apexes ofthe triangle are 1/2Li₂A, P⁴⁺A₂, and 1/2P⁵⁺2A₅. Incidentally, A is amixture of S and O, and A=S_(4-z/4)O_(z/4). In this ternary diagram, xis a factor relating the ratio of pentavalent P (P⁵⁺), and will beplotted closer to the apex 1/2P⁵⁺ ₂A₅ (lower right of the ternarydiagram), as x increases. Also, y is a factor relating the ratio oftetravalent P (P⁴⁺), and will be plotted closer to the apex P⁴⁺A₂ (lowerleft of the ternary diagram), as y increases. The composition ratio(4-4y-x) of Li is determined by the relationship between x and y.

The sulfide solid electrolyte in the present disclosure is representedby the composition formula Li_(4-4y-x)P⁴⁺ _(1+y-x)P⁵⁺ _(x)S_(4-z)O_(z)(═Li_(4-4y-x)P_(1+y)S_(4-z)O_(z)), wherein 0.6≤x<1, 0≤z≤0.2, and−0.025≤y≤0.1. In other words, a feature of the sulfide solid electrolytein the present disclosure is that it includes a tetravalent P. Forreference, Patent Literature 4 discloses a sulfide solid electrolyteincluding the composition formula Li_(3+5x′)P_(1-x′)S_(4-z′)O_(z′);however, this is plotted in the ternary diagram having Li₂S, Li₂S₅, andP₂O₅ at the apexes, includes a pentavalent P and does not include thetetravalent P. When it is plotted in the triangular prism of FIG. 1, itis plotted on the side surface at the right side of the triangularprism, and it is not the composition inside of the triangular prism.That is, the sulfide solid electrolyte in the present disclosure has acomposition different from that of, for example, Patent Literature 4.

Also, a feature of the sulfide solid electrolyte in the presentdisclosure is that it includes oxygen (O), that is, it includes anoxide. The oxide is generally excellent in chemical stability so thatimproves the chemical stability of the sulfide solid electrolyte in thepresent disclosure. Also, the sulfide solid electrolyte has a peculiarcrystal structure, and is thought that it obtains high ion conductivitydue to the peculiar crystal structure such as a tunnel structure throughwhich an ion is able to pass. When a part of S in the crystal structureis substituted with the oxygen (O), an ion conductivity improving effectmay be expected due to the shape change of the tunnel that contributesto the ion conductivity. A range of factor z relating the oxygen contentis in a range of 0≤z≤0.2. However, although the above mentioned effectis generally higher as the content of the oxygen (O) increases, when thecontent of the oxygen (O) is too much, a desired crystal structure maynot be obtained in some cases. Accordingly, the lower limit of factor zrelating the content of the oxygen (O) may be preferably more than 0,more preferably 1 or more, and further preferably more than 1. The upperlimit of factor z may be preferably less than 2, more preferably 1.8 orless, or less than 1.8, and further preferably 1.5 or less, or less than1.5.

The sulfide solid electrolyte in the present disclosure is representedby the composition formula Li_(4-4y-x)P⁴⁺ _(1+y-x)P⁵⁺ _(x)S_(4-z)O_(z)(═Li_(4-4y-x)P_(1+y)S_(4-z)O_(z)), wherein 0.6≤x≤1, 0≤z≤0.2, and−0.025≤y≤0.1. In this composition range, a sulfide solid electrolytehaving various crystal structures has been confirmed, and due to therespective crystal structure, excellent ion conductivity was alsoconfirmed. The sulfide solid electrolyte in the present disclosure mayinclude a LGPS type crystal structure, an α type crystal structure(crystal structure found in α phase of Li₃PS₄), and a β type crystalstructure (crystal structure found in β phase of Li₃PS₄) in some cases.These crystal structures are conventionally known to have ionconductivity, and the sulfide solid electrolytes having the crystalstructure are also expected to have ion conductivity. Further,surprisingly, in the above described composition range in the presentdisclosure, a new crystal structure different from the conventionallyknown crystal structures was also found, and the ion conductivitythereof was also confirmed. The followings will explain in each crystalstructure. Incidentally, respective crystal structure is capable ofbeing identified by the peak position in X-ray diffraction measurementusing a CuKα ray. The sulfide solid electrolyte in the presentdisclosure may be in a multiphase state in which a plurality of crystalstructures coexist. For example, the composition in the presentdisclosure may include a LGPS type crystal structure and an α typecrystal structure, and overlaps of these peaks may be detected in X-raydiffraction measurement.

The sulfide solid electrolyte in the present disclosure may include thecrystal structure of a LGPS type sulfide solid electrolyte. Also, thesulfide solid electrolyte may have a peak at a position of29=29.58°±0.50° in X-ray diffraction measurement using a CuKα ray; andthe sulfide solid electrolyte may not have a peak at a position of2θ=27.33°±0.50° in X-ray diffraction measurement using a CuKα ray, orwhen the sulfide solid electrolyte has a peak at the position of2θ=27.33°±0.50°, a diffraction intensity of the peak at 2θ=29.58°±0.50°is regarded as I_(A), and a diffraction intensity of the peak at2θ=27.33°±0.50° is regarded as I_(B), a value of I_(B)/I_(A) may be lessthan 0.50. This peak is found in the crystal structure of the LGPS typesulfide solid electrolyte, and has excellent ion conductivity.

The specification of I_(B)/I_(A) will be explained. The LGPS typesulfide solid electrolyte sometimes includes those other than thecrystal structure of the LGPS type having high ion conductivity, and forexample, sometimes includes a crystal phase having a peak in thevicinity of 2θ=27.33°. The ion conductivity of the crystal phase havinga peak in the vicinity of 2θ=27.33° is not high. Therefore, in thesulfide solid electrolyte in the present disclosure, in order todistinguish from the sulfide solid electrolyte having low ionconductivity, the value of I_(B)/I_(A), when the diffraction intensityof the peak in the vicinity of 2θ=29.58° is regarded as I_(A) and thediffraction intensity of the peak in the vicinity of 2θ=27.33° isregarded as Is, is specified to be less than 0.50. From the viewpoint ofthe ion conductivity, the proportion of the crystal phase having highion conductivity (having a peak at the position of 2θ=29.58°) in thesulfide solid electrolyte in the present disclosure is preferably high.Therefore, the value of I_(B)/I_(A) is preferably lower, andspecifically, is preferably 0.45 or less, more preferably 0.25 or less,further preferably 0.15 or less, and particularly preferably 0.07 orless. Also, the value of I_(B)/I_(A) is preferably 0. In other words,the sulfide solid electrolyte in the present disclosure does notpreferably have a peak in the vicinity of 2θ=27.33°. The sulfide solidelectrolyte in the present disclosure may be a solid electrolyte havingexcellent ion conductivity, when the proportion of the crystal phasehaving a peak in the vicinity of 2θ=29.58° is high.

Here, the peak position of 2θ=29.58° is an actual measured value, andthe peak position may slightly vary from 2θ=29.58° due to the slightchange of the crystal lattice according to, for example, the materialcomposition. Accordingly, the peak is defined as the peak at theposition of 29.58°±0.50°. Since the LGPS type sulfide solid electrolytehaving high ion conductivity is usually thought to have the peaks at2θ=17.38°, 20.18°, 20.44°, 23.56°, 23.96°, 24.93°, 26.96°, 29.07°,29.58°, 31.71°, 32.66°, and 33.39°, the sulfide solid electrolyte in thepresent disclosure also may have these peaks. Incidentally, these peakpositions also may vary in a range of t 0.50° in some cases.

Meanwhile, as described above, the peak in the vicinity of 2θ=27.33° isone of the peak of the crystal phase having low ion conductivity. Here,2θ=27.33° is an actual measured value, and the peak position mayslightly vary from 2θ=27.33° due to the slight change of the crystallattice according to, for example, the material composition.Accordingly, the peak of the crystal phase having low ion conductivityis defined as the peak at the position of 27.33°±0.50°. The crystalphase having low ion conductivity is usually thought to have the peaksat 2θ=17.46°, 18.12°, 19.99°, 22.73°, 25.72°, 27.33°, 29.16°, and29.78°. Incidentally, these peak positions also may vary in a range of±0.50° in some cases.

The sulfide solid electrolyte in the present disclosure may include an αtype crystal structure (crystal structure found in α phase of Li₃PS₄).Also, the sulfide solid electrolyte may have a peak at a position of2θ=17.90°±0.20°, 29.0°±0.5°, and 29.75°±0.25° in X-ray diffractionmeasurement using a CuKα ray; and when a diffraction intensity of thepeak at 2θ=17.90°±0.20° is regarded as I_(C), and a diffractionintensity of the peak at 2θ=18.50°±0.20° is regarded as I_(D), a valueof I_(D)/I_(C) may be less than 0.50. This peak is found in the α typecrystal structure (crystal structure found in α phase of Li₃PS₄), andhas excellent ion conductivity and excellent electrochemical stability.

The specification of I_(D)/I_(C) in relation to the α type crystalstructure will be explained. Although not desiring to be bound by aspecific theory, the peak of I_(C) is one factor of the distinguishingpeak of the solid electrolyte having the α type crystal structure, andthe crystal structure generating this peak I_(C) is thought to berelated to the ion conductivity and the chemical stability. In otherwords, when the peak of I_(C) is more clear, the crystal structureexcellent in the ion conductivity and the electrochemical stability isthought to be formed. When the peak of I_(D) (in a range of2θ=18.50°±0.20°) exists in relatively vicinity of I_(C), the crystalstructure generating the peak of I_(D) is formed, and the crystalstructure generating the peak of I_(C) is not likely to be formedrelatively so that the ion conductivity and the electrochemicalstability are thought to be deteriorated. Accordingly, from theviewpoint of the ion conductivity and the electrochemical stability, thevalue of I_(D)/I_(C) in the sulfide solid electrolyte in the presentdisclosure is preferably lower. Specifically, it is preferably 0.4 orless, more preferably 0.3 or less, more preferably 0.2 or less, andfurther preferably 0.1 or less. Also, the value of I_(D)/I_(C) ispreferably 0. In other words, the sulfide solid electrolyte having thisα type crystal structure does not preferably have a peak in a range of2θ=18.50°±0.20° that is the peak position of I_(D).

The sulfide solid electrolyte in the present disclosure may include anew crystal structure A different from the conventionally known crystalstructures. The new crystal structure A has a peak at a position of2θ=18.0°±0.1°, 19.4°±0.1°, 21.9°±0.1°, 24.0°±0.1° and 31.3°±0.1° inX-ray diffraction measurement using a CuKα ray. This is different fromthe above described LGPS type crystal structure, α type crystalstructure (crystal structure found in α phase of Li₃PS₄), and a R typecrystal structure (crystal structure found in $ phase of Li₃PS₄). FIG.2A shows a comparison of the X-ray diffraction peak found in the newcrystal structure A with the X-ray diffraction peak found in the Li—P—Sbased solid electrolyte conventionally know to have a crystal structure.From this comparison, it is clear that the new crystal structure A isdifferent from the conventionally know crystal structures. The newcrystal structure A has a large peak particularly at the position of19.4°±0.1°, and the peaks other than that are relatively small. Evenwhen the new crystal structure A is multiphased with other crystalstructures so that the small peak of the new phase A is immersed in thepeaks of other crystal structures, if the peak of 19.40±0.1° clearlyproject, the existence of the new phase is suggested.

Also, the sulfide solid electrolyte in the present disclosure mayinclude a new crystal structure B different from the conventionallyknown crystal structures. The new crystal structure B has a peak at aposition of 2θ=17.8°±0.1°, 19.1°±0.1°, 21.7°±0.1°, 23.8°±0.1° and30.85°±0.1° in X-ray diffraction measurement using a CuKα ray. This isdifferent from the above described LGPS type crystal structure, α typecrystal structure (crystal structure found in α phase of Li₃PS₄), a ptype crystal structure (crystal structure found in 1 phase of Li₃PS₄),and new crystal structure A. FIG. 2B shows a comparison of the X-raydiffraction peak found in the new crystal structure B with the X-raydiffraction peak found in the Li—P—S based solid electrolyteconventionally know to have a crystal structure. From this comparison,it is clear that the new crystal structure B is different from theconventionally know crystal structures. The new crystal structure B hasa large peak particularly at the position of 19.1°±0.1°, and the peaksother than that are relatively small. Even when the new crystalstructure B is multiphased with other crystal structures so that thesmall peak of the new phase B is immersed in the peaks of other crystalstructures, if the peak of 19.1°±0.1° clearly project, the existence ofthe new phase is suggested.

The sulfide solid electrolyte in the present disclosure may have variouscrystal structures, and is expected to have high ion conductivity. Theion conductivity of the sulfide solid electrolyte in the presentdisclosure may be preferably 0.4 mS/cm or more, more preferably 0.5mS/cm or more, further preferably 0.6 mS/cm or more, more preferably 0.7mS/cm or more, more preferably 0.8 mS/cm or more, further preferably 0.9mS/cm or more, and more preferably 1.0 mS/cm or more.

The measurement of the ion conductivity may be carried out in thefollowing manner.

A pellet is produced by charging the ground sample into a cell forsintered pellet, and then, applying the pressure of approximately 169MPa to a cell for normal temperature. Then, a sintered pellet includingthe solid electrolyte material of various compositions is obtained bysintering for 12 hours at 550° C. The sample for measuring is producedso as the radius of the pellet is approximately 10 mm and the thicknessis in a range of 1 mm to 2 mm. Au electrodes are stuck together to thesample for measuring to obtain a battery of Au/sample for measuring/Au.A Frequency Response Analyzer manufactured by NF Corporation is used formeasuring the ion conductivity of the sample for measuring. Thealternating current impedance measurement is carried out to measure theion conductivity of the sample under the following conditions: themeasuring range of 15 MHz to 100 Hz, the measuring temperature of 26° C.to 127° C., the alternating voltage of 50 mV to 100 mV, and theintegrating time of 2 seconds.

Since the solid electrolyte material according to the present disclosurehas high ion conductivity and high chemical stability, it may be used inany use application requiring ion conductivity and chemical stability.Among the above, the solid electrolyte material according to the presentdisclosure is preferably used for a battery. This is because it maycontribute to the improvement of the battery power greatly. Also, thesolid electrolyte material according to the present disclosure is amaterial including at least a sulfide composition (sulfide solidelectrolyte), may include nothing but the sulfide composition (sulfidesolid electrolyte), and may further include other compound (such as abinder).

The method for producing the solid electrolyte material according to thepresent disclosure will be explained. The method for producing the solidelectrolyte material according to the present disclosure is a method forproducing the solid electrolyte material according to the abovedescribed present disclosure, the method including: an ion conductivematerial synthesizing step of synthesizing an ion conductive materialusing a simple substance of P, a P compound, a S compound, a Licompound, and an O compound as a raw material including a constituent ofthe sulfide composition; and a heating step of obtaining the sulfidecomposition by heating the ion conductive material.

In the present disclosure, a simple substance of P, a P compound, a Scompound, a Li compound, and an O compound are used as raw materials.The simple substance of P is a pure phosphorus, and the valence of Phere is zero valent (P⁰). The P compound may be an oxide (such as P₂O₅),a sulfide (such as P₂S₅), or a phosphorus oxide (such as Li₃PO₄ andH₃PO₄), and the valence of P here is pentavalent (P⁵⁺). The S compoundis a sulfide, may be a sulfide or a sulfate of other raw materialelement, and may be, for example, P₂S₅, Li₂S, or Li₂SO₄. The Li compoundmay be an oxide, a sulfide, or a phosphate of other raw materialelement, and may be, for example, Li₂O, Li₂S, Li₂SO₄, or Li₃P⁵⁺O₄. The Ocompound may be an oxide of the other raw material element, and may be,for example, Li₂O, Li₂SO₄, Li₃P⁵⁺O₄, or P⁵⁺ ₂O₅.

Here, the phosphorus (P) whose valence is zero valent and the phosphorus(P) whose valence is pentavalent are used as the raw materials in thepresent disclosure. The oxidation-reduction reaction between thepentavalent P and the zero valent P occurs (the pentavalent P isoxidized whereas the zero valent P is reduced) in the ion conductivematerial synthesizing step and in the heating step, as the result, thequatrovalent P (P⁴⁺) occurs. By this, the sulfide solid electrolyte(sulfide composition) in the present disclosure includes thequatrovalent P.

Each raw material is preferably used according to the composition ratioso that the sulfide solid electrolyte has a desired composition formula.

The ion conductive material synthesizing step will be explained. In theion conductive material synthesizing step, the crystallinity of the rawmaterial is firstly deteriorated by micronizing the raw material by themechanical milling. By once deteriorating the crystallinity of thecrystalline raw material, the environment in which the sulfide solidelectrolyte with the crystal structure having high electrochemicalstability and high ion conductivity is likely to be deposited may beprovided. The micronizing is desirably carried out to an extent that thedesired peak of the raw material is made broad enough so that theenvironment in the end objective sulfide solid electrolyte will be theenvironment in which the crystal phase having the desired peak is likelyto be deposited. All of the raw material may be micronized, whereas onlya portion may be micronized. Particularly, it is preferable to micronizea compound (such as Li₂S) including the Li element. The compoundincluding the Li element has high crystallinity in many cases, and theremaining of such crystalline Li compound may possibly inhibit thedeposition of the end objective sulfide solid electrolyte.

Mechanical milling is a method of grinding a raw material while applyinga mechanical energy thereto. The raw material is micronized todeteriorate the crystallinity thereof by applying a mechanical energy tothe raw material. Examples of such mechanical milling may includevibrating mill, ball mill, turbo mill, mechano-fusion, and disk mill;above all, ball mill and vibrating mill are preferable.

The conditions of ball mill are not particularly limited if theconditions are such as to allow the micronized raw material to beobtained. Generally, larger number of revolutions brings highermicronizing rate, and longer treating time proceeds the micronization.The number of weighing table revolutions in performing planetary ballmill is in a range of 200 rpm to 700 rpm, for example, and preferably ina range of 250 rpm to 600 rpm, above all. Also, the treating time inperforming planetary ball mill is in a range of 1 hour to 100 hours, forexample, and preferably in a range of 1 hour to 70 hours, above all.Particularly, in order to sufficiently micronizing the compound (such asLi₂S) including the Li element, it is preferable to micronize by theball mill for 10 hours to 40 hours.

The conditions of vibrating mill are not particularly limited if theconditions are such as to allow the micronized raw material to beobtained. The vibration amplitude of vibrating mill is in a range of 5mm to 15 mm, for example, and preferably in a range of 6 mm to 10 mmabove all. The vibration frequency of vibrating mill is in a range of500 rpm to 2000 rpm, for example, and preferably in a range of 1000 rpmto 1800 rpm above all. The filling factor of a sample of vibrating millis in a range of 1% by volume to 80% by volume for example; above all,preferably in a range of 5% by volume to 60% by volume, and particularlypreferably in a range of 10% by volume to 50% by volume. Also, avibrator (such as a vibrator made of alumina) is preferably used forvibrating mill. Generally, vibrating mill is inferior to ball mill ingrinding efficiency; however, since a compound (such as P₂O₅ and P₂S₅)including the P element is micronized easily compared to the compound(such as Li₂S) including the Li element, the micronization by vibratingmill is suitable. The compound (such as P₂O₅ and P₂S₅) including the Pelement may be sufficiently micronized even by vibrating mill treatmentfor approximately 30 minutes.

Also, since the simple substance of P (pure phosphorus) is furthereasily micronized, it may be mixed by hand for approximately 5 minutes.

Next, the amorphized ion conductive material is synthesized by mixingthe micronized raw materials.

The ion conductive material is obtained by weighing and mixing the rawmaterial so that the composition is in the above describes preferablecomposition range.

The amorphized ion conductive material may be synthesized by firstlymixing each micronized raw material by hand, and further, sufficientlymixing by machinery mixing such as ball mill. The various mechanicalmilling used in the micronization may be used as a machinery mixingmethod, under the similar conditions. In addition to the micronization,by also utilizing mechanical milling in synthesizing, the amorphized ionconductive material may be synthesized by further deteriorating thecrystallinity of the raw material and mixing the raw materials uniformlywith each other. For sufficiently mixing, it is preferable tomicronizing by ball mill for 10 hours to 40 hours.

The heating step will be explained. The heating step is a step ofobtaining the sulfide solid electrolyte according to the presentdisclosure by heating the amorphized ion conductive material. Thecrystallinity is improved by heating the amorphized ion conductivematerial.

The heating temperature is not particularly limited as long as thedesired sulfide solid electrolyte may be obtained at the temperature,and the temperature is preferably higher than the temperature at whichthe sulfide solid electrolyte is crystallized. Specifically, the heatingtemperature is preferably 230° C. or more, more preferably 240° C. ormore, further preferably 250° C. or more, and further more preferably260° C. or more. Meanwhile the heating temperature is preferably low aspossible in light of workability and safety, and specifically, ispreferably 500° C. or less, more preferably 400° C. or less, furtherpreferably 350° C. or less, and further preferably 300° C. or less.Incidentally, although Patent Literatures 4 and 5 disclose Li—P—S basedsulfide solid electrolyte material including an oxygen (O), they add amelting step at 615° C. or more (Patent Literature 4) or at 550° C. ormore (Patent Literature 5), and compared to these, the sulfide solidelectrolyte may be obtained easily in the method for producing accordingto the present disclosure.

Also, the heating time is preferably adjusted appropriately so that thedesired sulfide solid electrolyte may be obtained. The heating time forobtaining the sulfide solid electrolyte according to the presentdisclosure may be approximately 4 hours, and the sulfide solidelectrolyte may be obtained easily. Further, when cooled to the roomtemperature after the heating, natural cooling may be employed, orannealing may be carried out in order to obtain the desired sulfidesolid electrolyte.

In a series of steps for producing the solid electrolyte material, theoperations are preferably carried out under an inert gas (such as Ar)atmosphere, in order to prevent the deterioration of the raw materialand the obtained solid electrolyte material due to moisture in the air.

EXAMPLES

Hereinafter, the present disclosure will be described in more detailswith reference to Examples. Incidentally, the following Examples do notlimit the present disclosure.

<Production of Li_(4-4y-x)P⁴⁺ _(1+y-x)P⁵⁺ _(x)S_(4-z)O_(z) Based Sample>

In a glove box under an argon atmosphere, Li₂S, P₂S₅, P₂O₅, and P (purephosphorus) were prepared as starting materials. Li₂S was micronized at380 rpm by ball mill for 10 hours to 40 hours, P₂S₅ and P₂O wererespectively micronized by vibrating mill for 30 minutes, P (purephosphorus) was ground (micronized) by hand for 5 minutes, and wereweighed. A mixed sample was prepared by mixing the micronized rawmaterials by hand for 5 minutes, and further mixing at 380 rpm by ballmill for 40 hours. A pellet of ϕ 13 mm was formed by charging the mixedsample into a pelleter and applying the pressure of 20 MPa to thepelleter using an uniaxial pressing machine. This pellet was sealed intoa carbon-coated quartz tube under nearly vacuum of 10 Pa. Then, thetemperature of the quartz tube including the pellet was elevated to 260°C. in 2 hours, the temperature was maintained for 4 hours, and then, wasnaturally cooled. Further, the resultant was ground for the evaluationthereafter.

The composition of the synthesized sample was plotted in the ternarydiagram in FIGS. 3 to 5 with a mark of X. Each ternary diagram in FIGS.3 to 5 is a magnification of that cut out from the composition diagramof a triangular prism shape in FIG. 1, at z=0 (FIG. 3), z=0.1 (FIG. 4),and z=0.2 (FIG. 5). In the vicinity of the plotted X, the number of thesample (such as S01), a main crystal structure, and an ion conductivityσ (mS/cm) were shown together. Also, in each FIGS. 3 to 5, guide linesof x and y relating the composition ratio in the composition formulaLi_(4-4y-x)P⁴⁺ _(1+y-x)P⁵⁺ _(x)S_(4-z)O_(z) were shown together.

The following measurement and evaluation were carried for the obtainedsamples.

<Powder X-Ray Diffraction Measurement>

In order to identify the crystal included in the produced samples apowder X-ray diffraction measurement was carried out by using a powderX-ray diffraction apparatus Ultima-IV (manufactured by RigakuCorporation) and Smart Lab (manufactured by Rigaku Corporation). For thepowder X-ray diffraction measurement, a Cu-Kα ray having the X-raywavelength of 1.5418 angstrom was used. The powder X-ray diffractionmeasurement was carried out in a range of 10° to 35° and at stepintervals in diffraction angle (2θ) of 0.01°.

<Ion Conductivity Measurement of Sintered Pellet>

A pellet was produced by charging the ground sample into a cell forsintered pellet, and then, applying the pressure of 169 MPa to a cellfor normal temperature. Then, a sintered pellet including the solidelectrolyte material (sulfide solid electrolyte) of various compositionswas obtained by sintering for 12 hours at 550° C. A sample for measuringwas produced so as the radius of the pellet was approximately 10 mm andthe thickness was in a range of 1 mm to 2 mm. Au electrodes were stucktogether to the sample for measuring to obtain a battery of Au/samplefor measuring/Au. A Frequency Response Analyzer manufactured by NFCorporation was used for the measuring the ion conductivity of thesample for measuring. The alternating current impedance measurement wascarried out to measure the ion conductivity of the sample under thefollowing conditions: the measuring range of 15 MHz to 100 Hz, themeasuring temperature of 26° C. to 127° C., the alternating voltage of50 mV to 100 mV, and the integrating time of 2 seconds. Also, asComparative Examples, investigations were carried out for the ionconductivity when the solid electrolyte materials that are differentfrom the solid electrolyte material according to the present disclosurewere used.

[Evaluation]

<Powder X-Ray Diffraction>

By using the solid electrolyte material plotted in the ternary diagramsin FIGS. 3 to 5, an X-ray diffraction (XRD) measurement was carried out.A part of the results thereof are shown in FIGS. 6 to 8. FIGS. 6 to 8are respectively X-ray diffraction of the composition of z=0, z=0.1, andz=0.2. A peak was confirmed in the solid electrolyte material having anyone of these compositions, and the possession of a crystal structure wassuggested. As z increases from 0, that is, as the oxygen (O) amountincreases, a tendency of the crystal structure to change from the J typecrystal structure to the α type crystal structure, and further to changeto the LGPS type crystal structure, was confirmed. FIG. 9 shows X-raydiffraction peaks of each composition, when z was varied from 0 to 0.5in the composition of Li₃PS_(4-z)O_(z) next to each other; the peak ofthe R type crystal structure was found when z=0 and 0.1, the peak of theα type crystal structure was found when z=0.2, and the peak of the LGPStype crystal structure was found when z=0.5. FIG. 10 shows X-raydiffraction peaks of each composition, when z was varied from 0 to 0.2in the composition of Li_(3.2)P_(0.96)S_(4-z)O_(z) next to each other;the peak of the β type crystal structure was confirmed when z=0, whereasthe peak of the LGPS type crystal structure was confirmed when z=0.2.

Further, a new crystal structure A, different from the conventionallyknown crystal structures, was also found. FIG. 11 shows the X-raydiffraction peak result of the new phase A and the peaks of a knowncrystal structures next to each other. The distinguishing peaks of thenew phase A were found at the positions of 2θ=18.0°±0.1°, 19.4°±0.1°,21.9°±0.1°, 24.0°±0.1° and 31.3°±0.1°. Particularly, the largest peakwas located at 19.4°±0.1°, and it was confirmed that it is a peak due tothe new crystal structure of the sulfide solid electrolyte by referringto the later described ion conductivity results, for example.

FIG. 12A shows the results of the X-ray diffraction measurement of thesamples synthesized additionally for replicating tests to confirm thenew phase A. Incidentally, samples were produced in Additional Examples1 and 2 in the same manner as the above described sample productionexcept that the composition was Li_(3.2)P_(0.975)S_(3.9)O_(0.1) and thatthe heating temperature was 260° C. in Additional Example 2. Both of theX-ray diffraction charts in Additional Examples 1 and 2 had fivedistinguishing diffraction peaks, and the expression of the new phase Awas confirmed again.

TABLE 1 Additional Example No. Composition Ball mill Burning 1Li_(3.2)P_(0.975)S_(3.9)O_(0.1) 380 rpm for 40 hours 240° C. for 4 hous2 Li_(3.2)P_(0.975)S_(3.9)O_(0.1) 380 rpm for 40 hours 260° C. for 4hous

Further, samples having the following compositions were synthesized inAdditional Examples 3 to 5 under the following production conditions,and the X-ray diffraction measurements were carried out for the samples.The results are shown in FIG. 12B. Every X-ray diffraction chart inAdditional Examples 3 to 5 had five distinguishing diffraction peaks,and the expression of the new phase B was confirmed.

TABLE 2 Additional Example No. Composition Ball mill Burning 3Li_(3.2)PS_(3.85)O_(0.15) 380 rpm for 40 hours 260° C. for 4 hous 4Li_(3.3)PS_(3.9)O_(0.1) 380 rpm for 40 hours 280° C. for 4 hous 5Li_(3.2)PS_(3.9)O_(0.1) 380 rpm for 40 hours 280° C. for 4 hous

<Ion Conductivity of Sintered Pellet>

The ion conductivity of the sintered pellets, obtained by sintering thepowder of the obtained Li_(4-4y-x)P⁴⁺ _(1+y-x)P⁵⁺ _(x)S_(4-z)O_(z) basedsamples, were measured at 26° C. to 127° C. FIG. 13 is a diagram showingthe above described ion conductivities together with X-ray diffractioncharts thereof. The ion conductivities a obtained by this were: in arange of 0.1 mS/cm to 0.4 mS/cm for the 0 phase (however, the one ofσ=0.1 is a composition including Br, and is a reference data out of thecomposition range according to the present disclosure), in a range of0.6 mS/cm to 1.3 mS/cm for the α phase, in a range of 0.6 mS/cm to 1.3mS/cm for the LGPS phase, and in a range of 0.8 mS/cm to 0.9 mS/cm forthe new phase A. Incidentally, the ion conductivity of the one havingthe new phase B shown in FIG. 2B was also measured, and was in a rangeof 1.2 mS/cm to 1.3 mS/cm. These are comparable to the ionconductivities reported for the conventional LGPS type solidelectrolytes.

What is claimed is:
 1. A solid electrolyte material comprising a sulfidecomposition represented by a composition formula Li_(4-4y-x)P⁴⁺_(1+y-x)P⁵⁺ _(x)S_(4-z)O_(z) (Li_(4-4y-x)P_(1+y)S_(4-z)O_(z)), wherein0.6≤x<1, 0≤z≤0.2, and −0.025≤y≤0.1.
 2. The solid electrolyte materialaccording to claim 1, wherein the solid electrolyte material has a peakat a position of 2θ=29.58°±0.50° in X-ray diffraction measurement usinga CuKα ray, and the solid electrolyte material does not have a peak at aposition of 2θ=27.33°±0.50° in X-ray diffraction measurement using aCuKα ray, or when the solid electrolyte material has a peak at theposition of 2θ=27.33°±0.50°, a diffraction intensity of the peak at2θ=29.58°±0.50° is regarded as I_(A), and a diffraction intensity of thepeak at 2θ=27.33°=0.50° is regarded as I_(B), a value of I_(B)/I_(A) isless than 0.50.
 3. The solid electrolyte material according to claim 1,wherein the solid electrolyte material has a peak at a position of2θ=17.90°±0.20°, 29.0°±0.50°, and 29.75°±0.25° in X-ray diffractionmeasurement using a CuKα ray, and the solid electrolyte material doesnot have a peak at a position of 2θ=18.50°+0.20° in X-ray diffractionmeasurement using a CuKα ray, or when the solid electrolyte material hasa peak at the position of 2θ=18.50°±0.20°, a diffraction intensity ofthe peak at 2θ=17.90°±0.20° is regarded as I_(C), and a diffractionintensity of the peak at 2θ=18.50°±0.20° is regarded as I_(D), a valueof I_(D)/I_(C) is less than 0.50.
 4. The solid electrolyte materialaccording to claim 1, wherein the solid electrolyte material has a peakat a position of 2θ=18.0°±0.1°, 19.4°±0.1°, 21.9°±0.1°, 24.0°±0.1° and31.3°±0.1° in X-ray diffraction measurement using a CuKα ray.
 5. Thesolid electrolyte material according to claim 1, wherein the solidelectrolyte material has a peak at a position of 2θ=17.8°±0.1°,19.1°±0.1°, 21.7°±0.1°, 23.8°±0.1° and 30.85°±0.1° in X-ray diffractionmeasurement using a CuKα ray.
 6. The solid electrolyte materialaccording to claim 1, wherein ion conductivity is 0.4 mS/cm or more. 7.A method for producing the solid electrolyte material according to claim1, the method comprising: an ion conductive material synthesizing stepof synthesizing an ion conductive material using a simple substance ofP, a P compound, a S compound, a Li compound, and an O compound as a rawmaterial including a constituent of the sulfide composition; and aheating step of obtaining the sulfide composition by heating the ionconductive material.
 8. The method according to claim 7, wherein aheating temperature in the heating step is in a range of 230° C. to 300°C.